Alignment targets are optical devices that are formed integrally on, or relative to, a wafer or die containing, for example, an integrated circuit. The purpose of the target is to provide positional information about the wafer or die to be used by processing equipment or tools during wafer processing. In targets, optical contrast is used to define the target. Typically, targets are configured with a central portion that optically contrasts with an outer portion located relative to the central portion that is arranged in some identifiable configuration. The contrast and/or the configuration are used to locate or identify the target using either contrast recognition or pattern recognition. This contrast is then used by optical systems to determine the position of the target, and in turn a position on the wafer or die relative to the target.
Two optical lighting configurations are often used in conjunction with optical targets, namely indirect lighting and direct lighting. In direct lighting systems, direct light is applied substantially normal (i.e., perpendicular) to the orientation of the target. In indirect lighting systems, indirect light is applied at some angle (usually a sharp angle from the plane on which the target is formed) to the surface. In many direct lighting systems and indirect lighting systems, the detector or observer is located substantially perpendicular to the surface where the light is directed.
Relatively large dies and relatively low circuit densities are quite adaptable for the placement of these optical targets, since the optical targets can be readily located on a die in areas not occupied by the circuitry. However, current trends in integrated circuits include decreasing the die size and increasing the circuit density. Therefore, the areas occupied by the optical targets can worsen layout and routing problems for the circuitry, for example, where the electronic circuitry has to be routed around the alignment targets. For these and other reasons, there is a need for the present invention.